This invention relates to devices for stitching closed the open ends of pliable containers such as thick walled or multi-layered paper bags that contain water softener salt, pet food and other dry products. More particularly, this invention concerns an apparatus and method for monitoring the feed velocity of thread to a stitching station to ensure that proper stitching is occurring. In applications utilizing closure tape to secure the open ends, this invention further concerns measuring the closure tape supply velocity to monitor its application to the containers.
Devices for stitching closed the open upper ends of upright bags are known, examples of such devices being found in U.S. Pat. No. 3,460,494 (Denker), and U.S. Pat. No. 3,782,069 (Fischbein). Such devices ordinarily employ a conveyor, carrying chains or other means to transport filled bags past a sewing head along the path traversed by the bags. At the sewing head, a needle forms stitches through the top portion of each bag. Many of these devices employ chain stitching, in which a sewing needle on one side of the bags operates on a first strand of thread, and a looper on the opposite side of the bags operates on a second strand of thread, in cooperation with the needle, to form a thread chain including thread from both strands. This process, called thread chaining with single or double thread, is described for example in the Denker patent.
A long recognized need in connection with such devices is thread monitoring, in order to ensure that proper stitching is taking place. One known approach is to employ a thread break detector, which responds to a total absence of thread movement to signal a fault or terminate operation of a sewing device. A significant problem with this approach, however, arises due to stitching faults caused by problems other than thread breakage.
More specifically, there can be excessive, or inadequate, tension in the needle thread caused by soiled or worn tensioner discs, or slippage of the thread from between the tensioner discs. A damaged needle can cause improper stitching. These faults, while altering thread supply speed, do not necessarily break the thread and thus can go undetected. The general concept of controlling stitching with respect to the feed-through speed of an item being stitched is known. For example, U.S. Pat. No. 3,654,882 (Kamena) discloses a control system that enables selection of the desired number of stitches per inch. An automatic embroidery device is shown in U.S. Pat. No. 4,254,724 (Conrads), in which an absolute position shaft encoder 1, driven by the sewing machine drive shaft, generates signals used to properly time various stitching actions. Also, to detect movement of cloth being sewn, U.S. Pat. No. 4,398,348 (Bergvall) discloses a toothed wheel 14 which engages the cloth and rotates as the cloth moves. A photoelectric detector senses wheel rotation.
While satisfactory in various specific applications, the above techniques do not adequately address the need for sensing the length of thread or other closure material consumed, in comparison to the linear travel of a container being closed, in order to ensure that proper closure is occurring.
Therefore, it is an object of the present invention to provide an apparatus and method for comparing the length of thread or other closure material used by a closure device in relation to the amount of linear travel of the containers being closed.
Another object is to provide an apparatus for monitoring thread consumption which is not time-dependent, and therefore automatically adjusts to changes in container transport speed.
Yet another object of the invention is to provide an apparatus for monitoring thread supply speed, in relation to container transport speed, without disturbing or interferring with the operation of the sewing device.